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The cold air return on my forced air furnace is too loud. While it is is tolerable, there is a noticeable sound of rushing air when the furnace is running (not a whistling sound, just a blowing sound).

I have a KeepRite G9MVE (80,000BTU 1600CFM).

I have read a couple of other posts an online resources, which generally seem to suggest:

  1. liners or baffles
  2. less restrictive grill
  3. larger duct size

For #2, I have removed the return air grill, but no change.

For #3, I removed the blower door and ran the furnace again. The return air was not noticeably quieter.

Using some of the calculations in the third link above, I think the 24"x6"=144sq.in trunk is probably too small (something like 900fpm flow), but then why would it not be significantly better with the blower door off? Doesn't this effectively bypass the return air duct?

Is there something I am missing here? How can I improve this issue?

My ducting looks something like this: HVAC overview

Here is the cold air return intake (A in diagram - 12"x30"=360sq.in) The intake runs between two pairs of floor joists, and the gap closest to the furnace, left side in photo, is noticeably louder than the other side. air intake

Here are the suspended trunks - return is on right, about 16' long (B in diagram - 24"x6"=144sq.in) trunks

Here is the furnace connection (filter 16"x25"=400sq.in) furnace

  • What's on the other side of the wall where the return grill is? Is relocating it a possibility at all? Bottom of the stairs is a horrible place for a floor grate, IMO. Some 'scoops' might have an effect, but I think not, if removing the grill doesn't stop it. – Mazura Feb 24 '16 at 4:48
  • That's an external wall. I agree that bottom of stairs is a horrible spot. But still I am not really looking for a major project, so I would rather not move it unless absolutely necessary. – Roberto Feb 24 '16 at 10:14
  • Can you describe the sound? – Tester101 Feb 24 '16 at 10:23
  • Sorry, that was probably an important detail to add to the question (I will edit). The sound is that of rushing air. Not whistling, or anything like that. – Roberto Feb 24 '16 at 10:43
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I'd have to say get an HVAC guy or firm. My first thought is that they could relocate this one under the stairs & have it suck through the side wall of the stair's stud bay. My 2nd thought is to add the stair stud wall as a 2nd smaller return, but then also move & shrink the floor one away from the stairs if there's another 4 or 6' away it could be.

The idea is that there should be a large sound reduction by getting that return out of the direct line of the HVAC fan's draw. You'd be getting rid of the direct sound path of the fan motor & fan, but also adding distance between you & the fan noise & splitting the suction whoosh in half. Definitely, see what they say. I don't see any of the other options doing anything, just like what you've tried & determined already.

  • I am certainly not against an HVAC guy. But I'd like to roughly know the answer first, and still do not feel I understand what is going on here. Does the "direct line" idea imply the duct size is OK, it's just the geometry that lets me hear the fan? The rushing air sound really makes me think this is an air velocity problem, I didn't think this was really "direct line" as there are a few feet and 90 degree bend to the main trunk, and another bend from the trunk into the furnace? But could be, given that removing the blower door (i.e. adding air volume) did not seem to make the problem go away. – Roberto Feb 24 '16 at 10:41
  • Yeah, I had a question about your diagram. It dawned on me after I answered that the diagram might be or is flipped, vents of the left in diagram are on right in pictures & the HVAC unit isn't under the stairs. – Iggy Feb 24 '16 at 11:26
  • The diagram is "flipped" compared to the photos, but you are right that the HVAC unit is under the stairs. Regardless, the return duct (neon green in the diagram) has some bends and offsets. – Roberto Feb 24 '16 at 11:32
  • Yeah, that's what I wasn't sure about. Putting returns into the walls, really all of them, would change the direction of their sound, the bigger plus. But, metal ducts carry sounds the best or worst in your situation, so more jogs & more return holes are the only way to reduce fan noise & whoosh concentrations. Otherwise, fiber ducts would isolate & deaden sound the best. You can try insulating the outside of the ducts yourself too. But, it's a very touchy & wasteful problem until you find the right combination of solutions. Going to Mitsubishi type Split-Systems would eliminate everything. – Iggy Feb 24 '16 at 11:37
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If it's not significantly better with the blower door off, while you cover the grate with a plastic garbage bag or something, what you're hearing is machine noise and IMO only solvable by my answer for #1.

But if the problem is whistling, your options are to lower the blower speed or enlarge the ducts. I see that yours is a newer unit, adjusting the blower speed lower may be an option. In fact, it may have been set high in the first place to push it though your ducts better.

If you do lower the speed you may be interested in my cut the gas back trick, so that it doesn't kick out on over heat.

Read this manual thoroughly and play with the SW1 switches (p. 13) to adjust it and make sure this manual lists your exact model number. It also includes the proper way to reduce the gas flow rate.

  • Just note that anything that says "increases comfort" probably drops efficiency. – Mazura Feb 24 '16 at 11:12
  • Thanks. I had previously seen that and tried different SW1 combinations. The furnace is currently set at 875CFM (0-0-1). But the SW1 settings did not seem to affect the sound or fan speed. The fan speed sections keep mentioning cooling only, so I am not sure the heating speed is adjustable? Table 5 (page 15) only gives one set of values for air volume vs static pressure (LOW vs HIGH heat), which also makes me think fan adjustment options are limited. – Roberto Feb 24 '16 at 11:20
  • OK, I will try a baffle first, as that seems easier. The duct liner would be needed in the whole duct (not just the return chamber)? I guess I would actually need to replace the duct, as the liner would shrink the height from 6" to 4" (or less)? – Roberto Feb 24 '16 at 11:28
  • @Roberto - Looks like if they're set at the defaults, it's already doing what we want. You might want to look at getting an ECM upgrade (infinitely variable blower motor, but I've never done one yet so...). - No, acoustical stuff wont help you if the problem is howling noise at the intake. – Mazura Feb 24 '16 at 11:33
  • Don't I have an ECM already? "Two−Stage, Variable Speed ECM Blower Motor". I think it means the fan speed varies to whatever is needed to get the desired CFM? So, for example, the CFM doesn't drop off just because the filter gets dirty (instead, the fan just spins faster)? – Roberto Feb 24 '16 at 11:38

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